(a) Field of the Invention
The present invention relates to an unleaded surface-treated steel sheet for fuel tanks of an automobile, more specifically, a surface-treated steel sheet coated with a chromate layer having excellent cosmetic corrosion resistance and fuel corrosion resistance as well as seam processibility and a method of fabricating the same.
(b) Description of the Related Art
Generally, it is required that fuel tanks for storing fuel of an automobile should have resistance to corrosion on its outer surface to be exposed to the atmosphere (hereinafter, referred to as the xe2x80x9ccosmetic corrosion resistancexe2x80x9d) as well as resistance to corrosion on its inner surface to contact fuel such as gasoline (hereinafter, referred to as the xe2x80x9cfuel corrosion resistancexe2x80x9d).
A fuel tank is usually made by press-forming steel sheets into cup-shaped upper and lower tank bodies, and welding the bodies to each other by resistance welding method such as spot welding or seam welding, or joining method such as soldering or brazing. In this respect, because two parts are joined each other in a fuel tank, good weldability and good processibility are required for the steel sheet to be applied for use in forming fuel tanks.
A ternesheet that is a cold-rolled steel sheet plated with lead-tin alloy has widely been used for fuel tanks. However, the ternesheet has been limited in its use because it contains lead that causes environmental pollution.
Korean Patent Application No. 97-703448 and Japanese Patent Laid Open Publication No. Hei9-59783 disclose unleaded surface-treated steel sheet that can replace ternesheet. The above inventions relate to coating special chromate solution on a zinc-nickel (Znxe2x80x94Ni) alloy plated steel sheet. However, the above technique requires a minute crack-format on process on the zinc-nickel alloy plated layer before treating chromate, thus the manufacturing process is complicated. Furthermore, in such a structure, chrome is liable to elute even in contact with minimum amount of moisture contained in fuel, and this results in deteriorated fuel corrosion resistance.
To solve these problems, the present inventors provided chromate solution for a surface-treated steel sheet that does not need to form minute cracks on the plated layer and can prevent elution of chrome in the international patent publication No. WO 00/32843.
As an automobile company using surface-treated steel sheet for fuel tank tries to develop the appearance of an automobile variously, necessity of seam processing increases when manufacturing fuel tanks. When fuel tanks are manufactured by seam processing, a coating layer of surface-treated steel sheet which is subjected to seam-process forms cracks and peels off as well.
In addition, recent automobile companies have a tendency to replace electroplated steel sheet with melted zinc plated steel sheet in order to reduce cost and to increase cosmetic corrosion resistance of an automobile. Melted zinc plated steel sheet, however, is not appropriate to use as a fuel tank of automobile because it has problems such as cracking or exfoliation at the coating during seam processing even if it has good cosmetic corrosion resistance.
Thus, there is a need for developing chromate solution and a process for treating the same that satisfy cosmetic corrosion resistance and fuel corrosion resistance as well as seam processibility at the same time.
Therefore, the present invention is derived to solve the above problems. The present invention provides an unleaded surface-treated steel sheet coated with chromate which prevents cracks of coating layer during seam processing thereby improving cosmetic corrosion resistance and fuel corrosion resistance after processing.
It is another object of the present invention to provide chromate solution used for manufacturing surface-treated steel sheet coated with chromate which can improve cosmetic corrosion resistance and fuel corrosion resistance after processing.
It is another object of the present invention to provide chromate solution that adheres into melted zinc plated steel sheet and can prevent cracks or exfoliation due to abrasion between press plane during seam processing.
It is another object of the present invention to provide a process for fabricating unleaded surface-treated steel sheet coated with chromate layer which has excellent cosmetic corrosion resistance and fuel corrosion resistance after processing.
In order to achieve the above objects, the present invention provides a resin solution comprising: (a) a main solution prepared by adding 50 to 80 w % of phosphoric acid, 20 to 40 w % of fluoric acid, 300 to 2000 w % of colloidal silica, 5 to 15 w % of sulfuric acid, and 25 to 40 w % of sodium phosphate on the basis of chrome in a chrome solution having 0.4 to 0.8 of trivalent chrome rate and 7 to 50 g/l of dissolved chrome; and (b) a hardener aqueous solution comprising one of epoxy silane or glycidyl silane of 2 to 10 w % on the basis of total hardener solution, the pH of which being controlled between 2 and 3, said hardener aqueous solution of 10 to 60 w % being added to the main solution. Wherein, pH of said hardener aqueous solution is controlled by phosphoric acid.
In order to achieve the above objects, the present invention provides a method of fabricating surface-treated steel sheet for a fuel tank of an automobile comprising the steps of: plating zinc/zinc-based alloy on surface of low carbon content steel; coating the chromate solution of claim 1 on steel sheet plated with zinc/zinc-based alloy so as to have 30-150 mg/m2 of chrome content of coating layer after drying; and baking said steel sheet coated with chromate solution at 140-250xc2x0 C. Wherein, said plating step comprises soaking zinc/zinc-based alloy into steel sheet in electroplated or melted state.
In order to achieve the above objects, the present invention provides an unleaded surface-treated steel sheet coated with chromate solution.
According to the present invention, it is possible to provide an unleaded surface-treated steel sheet coated with chromate solution which compactly adheres melted zinc plated layer or zinc/zinc-based alloy electric plated layer so that it can reinforce adhesion between steel sheet and coating layer while seam processing, and improve cosmetic corrosion resistance and fuel corrosion resistance even after processing.
An unleaded surface-treated steel sheet according to the present invention is prepared by electroplated zinc or zinc-nickel alloy, or plating melted zinc on cold-rolled steel sheet and coating chromate solution on the above steel sheet plated with eletroplating or melted zinc plating.
Low-carbon steel sheet having less than or equal to 0.03% of carbon content is used as a cold-rolled steel sheet in the present invention.
Hereinafter, chromate solution used for chromate layer coating is explained in detail.
Characteristics of chromate solution used in the present invention are as follows:
First, in order to maintain secure adhesion between coated chromate layer and zinc or zinc alloy layer, content of hardener solution added to the chromate solution is optimized. Thus, it can remarkably prevent destruction of chromate coating layer when processing surface-treated steel sheet coated with chromate layer. Accordingly, it is not necessary to form minute cracks on the surface of plated layer as prior art so that the process become simple and cost is reduced.
Second, excellent fuel corrosion resistance is obtained by drastically reducing elution of six-valent chrome while increasing hydrophobicity of chromate coating layer.
Third, since structure of chromate layer is improved by adding additives into chromate solution, the surface-treated steel sheet according to the present invention can maintain excellent cosmetic corrosion resistance and fuel corrosion resistance even after processing.
The chromate solution of the present invention comprises main solution and hardener solution.
The main solution comprises phosphoric acid, fluoric acid, colloidal silica, sulfuric acid and sodium phosphate as additives in addition to chrome aqueous solution, wherein sodium phosphate is added to improve structure of coating layer. Also, hardener solution comprises epoxy silane or glycidyl silane aqueous solution.
Chrome aqueous solution as a main component in the main solution is prepared as follows.
First, anhydrous chromic acid of 70-200 g/l is added into distilled water and is dissolved. Then, ethylene glycol is added to dissolved chromate and six-valent chrome is reduced to tri-valent chrome so that ratio of tri-valent chrome with respect to total chrome(reduction ratio) is 0.4-0.8. When reduction ratio is less than 0.4, it cannot obtain effect of cosmetic corrosion resistance by tri-valent chrome and content of elution chrome increases as soluble six-valent chrome increases. Meanwhile, when reduction ratio is more than 0.8, solution becomes gel so that storability of solution decreases.
Thus, concentration of chrome is set to 7-50 g/l by adding distilled water into chrome aqueous solution. The reason for limiting concentration of chrome relates to content of chrome added to a steel sheet. A roll coating process is used when chromate is coated on surface of plated steel sheet in the present invention. When the roll coating process is used, content of chrome adhering to steel sheet depends on rotation speed of a roll and pressure applied between rolls. Therefore, considering these characteristics, it needs to prepare solution which forms optimal coating layer. When content of chrome in chrome aqueous solution is less than 7 g/l, the amount of chrome adhering to steel sheet is deficient, whereas when more than 50 g/l, the appearance of the surface is not good and the stability of the solution decreases.
In order to improve material property of chromate solution prepared as above, phosphoric acid, fluoric acid, colloidal silica and sulfuric acid are added.
Content of phosphoric acid is preferably 50-80 wt % on the basis of chrome component. If phosphoric acid is less than 50 wt %, it is not sufficient to have improved effect of solution property and surface property. If phosphoric acid is more than 80 wt %, reduction ratio increases so that stability of solution decreases, and excess phosphate decreases cosmetic corrosion resistance.
Fluoric acid is added to improve cosmetic corrosion resistance and evenness of coating layer. If fluoric acid is added less than 20 wt % on the basis of chrome component, cosmetic corrosion resistance is not sufficient whereas if more than 40 wt %, sludge occurs in the solution so that stability of solution decreases. Therefore, content of fluoric acid is preferably 20-40 wt %.
Silicone oxide(SiO2) having pH 2-4 in the colloidal state as silica is added preferably in 300-2000 wt % on the basis of chrome component. Silica forms crosslink-bond during baking so that it prevents formation of zinc oxide at matrix metal. In addition, since silica is hydrophobic, it can increase cosmetic corrosion resistance against moisture and improve adhesion of coating layer. However, if silica is less than 300 wt %, the above effects are not sufficient whereas if silica is more than 2000 wt %, stability of solution and adhesion of coating decrease.
Sulfuric acid can improve surface quality of steel sheet by regulating color of solution. Also, it can improve stability of solution by improving flow of solution. If sulfuric acid is less than 5 wt % on the basis of chrome component, effect of material property of surface decreases whereas if more than 15 wt %, stability and cosmetic corrosion resistance of solution deteriorates.
The main solution of the present invention is prepared by adding the above element into chrome solution, further adding sodium phosphate as explained following, and mixing these.
The reason for addition of sodium phosphate is to improve material property of chromate coating layer. That is, sodium phosphate can prevent cracks of coating layer during seam processing so that it maintains cosmetic corrosion resistance and fuel corrosion resistance after processing. If sodium phosphate is less than 25wt %, effect of improving coating structure decreases. On the contrary, if sodium phosphate is more than 40 wt %, precipitation occurs in the solution so that stability of solution decreases. Therefore, content of sodium phosphate is preferably 25-40 wt %.
Chromate solution of the present invention is prepared by adding a hardener into the main solution prepared as above.
Hardener added to main solution facilitates cross link bond of chromate so that it forms chrome coating layer and simultaneously serves as maintaining adhesion of chromate coating layer and matrix metal. Thus, it is important to select hardener considering kind of zinc or zinc alloy plating which plated on steel sheet and the above effect. The hardener of the present invention includes epoxy silane and glycidyl silane considering the above.
First, a method of preparation for hardener solution comprising epoxy silane is as follows.
A hardener solution comprising epoxy silane is prepared by adding 2-10 wt % of epoxy silane into distilled water and adding small amount of phosphoric acid to prevent gelation when mixing with main solution. Thus, pH of hardener solution is set to be similar to that of main solution, which is 2-3. There are many method to set pH. Among those, it is preferable to add phosphoric acid. If epoxy silane is added less than 2 wt %, chrome cross-link reaction rarely occurs. If epoxy silane is added more than 10 wt %, storability of solution decreases.
If hardener solution comprising epoxy silane is added less than 10 wt % on the basis of main solution, cross-link reaction does not occur sufficiently. If hardener solution comprising epoxy silane is added more than 50 wt %, stability of solution decreases. Thus, content of hardener solution comprising epoxy silane is preferably 10-50 wt % on the basis of main solution.
Then, a method of preparation for hardener solution comprising glycidyl silane is as follows.
A hardener solution comprising glycidyl silane is prepared by adding 2-10 wt % of glycidyl silane into distilled water and adding small amount of phosphoric acid to prevent gelation when mixing with main solution. Thus, pH of hardener solution is set to be similar to that of main solution, which is 2-3. If glycidyl silane is added less than 2 wt %, content of hardener needed to form chromate cross-link bond increases and total content of chrome decreases so that amount of chrome added to steel sheet decreases during chrome coating. If glycidyl silane is added more than 10 wt %, storability of solution decreases so that precipitation occurs in the solution.
If hardener solution comprising glycidyl silane is added less than 20 wt % on the basis of main solution, cross-link reaction of chromate does not occur at an appropriate temperature and time. Thus, chromates become macromolecule and do not form coating layer. And then, while heating and cooling it, most of chrome elutes so that cosmetic corrosion resistance dramatically decreases. If hardener solution comprising glycidyl silane is added more than 60 wt %, hardener component of chromate solution is subjected to occur chromate reduction so that stability of solution decreases. Therefore, it is preferable to add 20-60 wt % of hardener solution comprising glycidyl silane on the basis of main solution.
Particularly, glycidyl silane added to hardener solution reinforces adhesion of plated layer and chromate coating layer while treating melted zinc plated steel sheet with chromate. This is because glycidyl silane increases reactivity with melted zinc plated layer, which prevents exfoliation of coating layer or elution of surface by penetration of solvent.
Chromate solution prepared as above can be used as steel sheet for fuel tank by coating chromate on zinc-based alloy electroplated steel sheet or melted zinc plated steel sheet. In addition, resin may be coated on the upper side of the chromate coating layer for a fuel tank.
Since zinc plated steel sheet of the present invention includes zinc-based alloy plated steel sheet such as Zn or Znxe2x80x94Ni, the present invention is not limited to zinc-based alloy exemplified in the specification.
Method of formation of chromate coating layer includes a reactive type, an electrolyte type, and a coating type. A reactive type and an electrolyte type, however, occur water pollution because they make six-valent chrome elution easily. Thus, a coating type is preferable in terms of cosmetic corrosion resistance and minimization of pollution.
Three-stage roll coater can be used for coating chromate solution on zinc/zinc-based alloy plated steel sheet. The amount of chromate attached to the steel sheet is regulated by each roll driving direction, rolling speed, and each roll adhesion pressure.
The amount of chromate is preferably 30-150 mg/m2 on the basis of dried coating content on the plated steel sheet. If the content is less than 30 mg/m2, cosmetic corrosion resistance is not sufficient whereas If the content is more than 150 mg/m2, it is not economical and chrome elutes to deteriorate characteristics of coating layer.
Baking temperature of the steel sheet coated with chromate is preferably 140-250xc2x0 C. If baking temperature is less than 140xc2x0 C., hardening reaction of coating layer is not sufficient. On the contrary, if the baking temperature is more than 250xc2x0 C., minute cracks occur on the surface so that cosmetic corrosion resistance decreases.
Now, preferred embodiments are suggested to help the apparent understanding of the present invention. The below embodiments are provided for the sake of clear understanding only and the present invention is not limited thereto.